In the past, fingerprints were traditionally taken by government law enforcement agencies such as the state's attorney's office, police officials and corrections officials for purposes of registration, identification and verification of suspected persons. Border controls and voter identification are more recent additions to such civilian applications.
Whereas the capture of fingerprints in criminal cases is still supervised and carried out by well-trained, experienced security officials, there are usually no technicians present during the capture and evaluation of fingerprints in civilian applications. Moreover, fingerprinting for civilian purposes is often carried out in high-security areas such as embassies, consulates and specific areas of airports where physical contact between the operator of the capture device and the person being fingerprinted is not permitted. Therefore, the person being fingerprinted must be instructed without the direct attendance of an operator of the capture device.
In order to simplify the fingerprinting process in this regard and to render subsequent evaluation of the quality of the fingerprints more objective, i.e., in order to eliminate subjective visual evaluation, fingerprint scanners were increasingly outfitted with feedback elements. Examples of such feedback elements are acoustic signal outputs (e.g., beepers, loudspeakers), visual displays such as pictograms, LEDs of one or more colors, or location-based displays. Some examples of fingerprint scanners or hand scanners outfitted in this way are the Guardian (manufactured by Cross Match Technologies), RealScan G10 (Suprema), CS500e (Cogent), and MorphoTop (Morpho). With all of these devices, however, it is assumed that the person whose fingerprints are to be taken is either instructed by a device operator or already has experience with a fingerprint scanner, so that the manner in which the fingers are placed is assumed to be correct and only the qualitative results of the individual impressions are displayed and improvements are requested if necessary.
While multicolored LED displays can show whether or not the image quality of a fingerprint is sufficiently good, they cannot provide any assistance as to the conditions which must be changed in order to achieve a sufficient quality of the fingerprints. Further, no instructions can be given as to which fingers are to be placed on the capture surface of the device or where to place the fingers.
Feedback elements for indicating print quality are disclosed, for example, in U.S. Pat. No. 7,277,562 B2, in which the display elements are triggered by evaluating units of the control software. Although the activity of the user is analyzed and results are displayed to demand certain further actions on the part of the user, these display elements are not adequate for guiding the user in an easily understandable manner.
A user guidance study conducted in 2007 by the NIST (National Institute of Standards and Technology, an arm of the U.S. Department of Commerce) confirmed that blinking LEDs were not helpful in the fingerprinting process and in some cases even irritated the user and led to delays in the capture process (Theofanos et al., NISTIR 7403, 2007). In addition, studies showed that video animations were most suitable for guiding users, particularly inexperienced users. However, for safety reasons and due to limited space, monitors capable of showing video instructions of this kind could often not be installed at fingerprinting stations. But even if it were possible to show instructional videos, the user would be compelled to concentrate on the fingerprint scanner for correct placement of fingers and simultaneously on the monitor for video instructions. In most cases, however, video instructions of this kind have no direct link with the actual capture process but are largely continuous loops without real-time feedback for the user with regard to deficiencies in the fingerprint capture.
Present-day local displays on fingerprint scanners or hand scanners help to instruct the user on the fingerprinting process; however, they offer feedback on proper acquisition of the scanned fingerprints only to the operator and not to the person whose fingerprints are being recorded. Examples of hand scanners having local displays of this kind are, e.g., L SCAN 500P (manufactured by Cross Match Technologies), RealScan F (Suprema), MultiScan 1000 (Green Bit) and LS 1100 (Secure Outcome).
The feedback information in local displays of this kind are live images of the fingerprints which are shown in real time as the fingerprints are captured. However, they do not give instructions to the user about how to improve fingerprints of insufficient quality. Further, an untrained user does not know how to position a finger for a usable fingerprint or when a displayed print is good or perfect. This can only be assessed by experienced fingerprint experts.
The live images of recorded fingerprints can also be displayed on the screens of computers which are linked to the fingerprint scanner for controlling. But screens of this kind are usually not visible to the person being fingerprinted or, if so, do not convey any comprehensible feedback to the untrained user about the position and quality of the fingerprint.
In modern fingerprint capture processes for non-criminal applications, i.e., when no official or operator physically carries out or supervises the fingerprinting, three important pieces of information which would allow untrained persons to capture fingerprints are lacking:
a) the correct positioning of the fingers;
b) the need to keep the fingers still during fingerprinting; and
c) the correct pressure with which to press the fingers on the capture surface.
Positioning:
The correct position of the fingers on the capture surface is important for ensuring that all fingerprints are completely visible in the final fingerprint image. No fingerprints may be cut off or overlap. In many cases, the fingers are (i) positioned too close together, (ii) spread out beyond the edges of the capture surface, or (iii) not placed in the center of the capture surface so that the fingerprints are not captured in their entirety.
Current fingerprint scanners do not give any feedback on faulty positioning of the fingers as described above, and software-generated arrows superposed on the displayed print image are difficult to comprehend or require more than minimal knowledge about the fingerprinting process. Such comprehension cannot be assumed in the case of persons whose fingerprints are to be recorded for civilian applications.
Holding Fingers Still:
Although the time required for capturing a fingerprint by means of a device with a fingerprint scanner has been significantly reduced in recent years through faster image sensors, it still takes 1 to 3 seconds to capture a set of four fingers. The fingers may not move during this time; otherwise details of the fingerprints (minutiae) will be distorted. At the present time, however, there are no means provided on fingerprint scanners for adequately drawing the attention of the user to this source of errors.
Correct Finger Pressure:
Sufficient contrast is one of the most important prerequisites for the ability to examine, compare and check fingerprints. Contrast in a fingerprint depends upon skin condition (damp/dry, tender/rough/young/old) as well as on the pressure by which the finger is pressed on the capture surface.
Even for trained law enforcement officers, the image quality of a fingerprint image captured by a live scanner (FBI and NIST standard) and displayed on a computer screen is difficult to evaluate by visual assessment. Therefore, in most cases the image quality is analyzed by software algorithms in a step subsequent to the capture of the print. One example of an algorithm of this kind is the algorithm developed by the National Institute of Standards and Technology (NIST) for assessing image quality NFIQ (NIST Fingerprint Image Quality). This NFIQ algorithm assesses fingerprints based on the probability that a sufficiently good comparability with other fingerprints is possible in automatic fingerprint comparison systems (AFIS, or Automatic Fingerprint Identification Systems). In this case, five levels are distinguished, where NFIQ level 1 is the poorest image quality and level 5 is the highest image quality. For operating personnel in civilian applications and particularly for persons whose fingerprints are to be captured, it is completely impossible to visually assess a live scanner fingerprint for evaluation of image quantity.
For live scan devices, as they are called, which are used in government identity management systems (police, passport, personal identification, voter registration, etc.) and which are mostly operated by trained personnel, existing requirements of all kinds regarding the fingers to be acquired (index finger, middle finger, ring finger, little finger, thumb), the quantity of fingers (one finger by itself, two fingers together, four fingers together), type of print (flat, rolled), capture format (e.g., 41×39 mm, 80×75 mm), image resolution (500 ppi or 1000 ppi) and quality parameters of these images (SNR, linearity, geometric distortion, gray value uniformity, MTF) are set very high, and compliance must be demonstrated within the framework of an official approval procedure (the FBI within the United States, the BSI within Germany, the STQC within India and the NPA within Japan). But even for these systems in the course of further commercialization for civilian applications (e.g., tourist traffic, issuing of visas, etc.), application of the invention is desirable and, moreover, leads to a more objective assessment of a fingerprint whose quality is high enough to be approved.
In principle, fingerprints have sufficiently high quality when there is in fact only one matching person determined from a database of several million entries with a very high probability in the automated process. Institutions such as the FBI and the NIST have derived technical parameters from this “top grade” quality which today constitute approval requirements for high-quality fingerprint reading devices.
Nevertheless, assigning a fingerprint to one single person from a prepared database (also a database having fewer entries and when using noncertified equipment) is the criterion for sufficiently high quality for the capture and checking of fingerprints, which means that the fingerprints must be captured in such a way that minutiae thereof are acquired completely and with as little distortion as possible by checking the above-mentioned parameters at least by a quick test.
Steps for improving the capture of fingerprints for reproducible acquisition of high-quality print images are described, for example, in U.S. Pat. Nos. 7,319,565 B2 and 5,416,573 A.
The simplest way to suitably adjust contrast before and during the capture process is to vary the pressing pressure. On the other hand, contrast problems occur in most cases precisely because of too much or too little finger pressure. Too much pressure leads to very dark images and it is no longer possible to separate ridges and valleys of the skin. Conversely, too little pressure leads to low image contrast, and the ridges of skin wrinkles are invisible or only partially visible.
The use of a simple pressure sensor cannot solve this problem satisfactorily because, as was mentioned above, the contrast also depends significantly upon skin condition.